This invention relates to an active matrix electro-optic display device such as a liquid crystal display (LCD) device, having an array of display cells defined by an array of pixel electrodes, an array of row and column electrodes, and an array of switches inter-connecting the pixel electrodes and the row and column electrodes.
Liquid crystal devices currently being developed for television applications are of the active matrix type in which an array of display cells is defined by an array of pixel electrodes on one wall of the device, and in which the display cells are controlled by a matrix of row and column electrodes connected to the pixel electrodes through switches, e.g., thin film transistors.
In one preferred form of such an active matrix liquid crystal display device, known as a twisted nematic display device, alignment layers are oriented on the inner walls of the device to have mutually orthogonal alignment directions, thereby aligning the adjacent molecules of a nematic liquid crystal and imparting a 90 degree twist to the liquid crystal from one wall to the other.
In operation, incoming light to be modulated enters the device through a polarizer having its polarization direction aligned with the adjacent alignment layer. This linearly polarized light is rotated 90 degrees as it travels through the liquid crystal, and exits the device through an analyzer having its polarization direction aligned with the adjacent alignment layer. Upon application of a voltage sufficient to untwist the liquid crystal molecules, the light is no longer rotated and is thus blocked by the analyzer, giving a dark "on" condition. Application of intermediate voltage values result in intermediate or "grey" levels.
One consideration in designing the pixel array is the amount of brightness which is desired for the display. Since the brightness is related to the amount of light transmitted by the device, it is important to minimize the area covered by non-transmissive features such as the switches and the row and column electrodes. The usual practice is to render the entire inter-pixel area non-transmissive in order to enhance the contrast of the display. The brightness of such displays is thus directly related to the percentage of the display area which is occupied by pixels.
Another consideration in designing the pixel array is the resolution desired for the display. Pixels in current displays are arranged in an orthogonal pattern ("manhattan layout") with the row address lines parallel to the x axis of the display and the column address lines parallel to the y axis (FIG. 1). In some displays, the rows may be staggered to produce a higher effective horizontal resolution (FIG. 2).
In Japanese patent laid open 63-142330, an active matrix liquid crystal display device is described which has a close-packed array of hexagonal-shaped pixels, said to have increased display area due to the smaller periphery per surface area of the hexagonal pixels over square or rectangular pixels, leading to shorter row and column lengths on the periphery of the pixels, as well as the elimination of longitudinal and transverse stripe-shaped artifacts which occur with square and rectangular pixels. However, the row and column lines are actually longer in this hexagonal design since they zigzag. Also, the increased area-to-periphery ratio advantage of this design is largely nullified because the row and column lines are parallel to each other over 1/3 of the periphery of each pixel.
In European patent application no. 0 302 653, an active matrix liquid crystal display device is described in which pixels of first, second and third colors are centered on the corners of first, second and third overlapping rectangles having a common diagonal, this arrangement said to provide the same display quality for horizontal, vertical and oblique lines, as well as the elimination of stripe-shaped artifacts, regardless of the color of the display.
In Japanese patent application no. 62-236171, which corresponds to Japanese patent laid open 64-79728, a liquid crystal display said to have high contrast has scanning (row) electrodes of a zigzag pattern and signal (column) electrodes of a straight pattern in combination with a rhombic pattern of picture elements.